Chemical Phenomena in Birch Species in Response to Natural Challenges

Keywords: Drought Adaptation, Betulin, Papyriferic Acid, Antioxidant Enzymes, Induced Systemic Resistance, Jasmonic Acid Signalling, Hydrophobicity, Methyl Salicylate

Abstract

Birch is a type of hardwood tree under the genus Betula, which belongs to the Betulaceae family. Within the genus of Betula, about 40 species of birch trees exist that are widespread across northern regions. Birch extracts have historically been used for a variety of medicinal and healing practices, and further research shows how remarkably unique and powerful these organic compounds are. This paper explores the particular chemical constituents of birch trees that allow them to thrive in conditions other trees cannot. Papyriferic acid functions as a sophisticated biochemical deterrent, protecting birches from herbivory through inhibition of succinate dehydrogenase, which impairs the herbivore’s energy metabolism. Birch also makes use of an organic molecule called betulin to help them increase stemflow to their roots and to protect from pathogens, fungi, and insects. Methyl salicylate is produced to help protect themselves against piercing/sucking herbivores and is also used as a signalling mechanism to call in predators. Birch has also biochemically adapted to drought and pathogen stress by accumulating osmoprotectants, antioxidant enzymes, and antimicrobial compounds. Moreover, it produces signalling hormones in response to stimuli and damage as an energy-efficient defense pathway.

Introduction

Birch trees are slender, fast-growing, thin-leaved deciduous trees characterized by their thin, papery, white bark. Papyriferic acid (PA), a triterpenoid resin compound, protects juvenile birches from browsing by snowshoe hares by uncompetitively inhibiting succinate dehydrogenase, disrupting energy metabolism in herbivores. They are typically short-lived and dominate northern temperate, boreal, and alpine climates. By fine-tuning their internal chemistry through osmoprotectant accumulation, antioxidant enzyme activation, and antimicrobial compound production, birch species maintain cellular stability and ward off infection under environmental stress. Birch trees are associated with wintery regions where soil is often frozen. Birch has evolved sophisticated biochemical responses to survive in environments challenged by drought and microbial threats. Their remarkable biochemical constitution allows birch forests to proliferate in harsh environments. Finally, birch trees feature inducible defense systems against feeding and other external stresses. In response to stimulus, special hormones are released that signal the production of a vast variety of chemicals that protect the tree and deter feeders.

Chemical Defense of Birches Against Snowshoe Hares

Birch trees are widespread across the boreal forests of North America, where they form an essential part of the forest ecosystem. Yet, the relationship between birch and herbivores like the snowshoe hare goes beyond simple feeding dynamics. Over time, birch trees have evolved a variety of chemical defenses that affect herbivore diet selection, particularly through the production of secondary metabolites in their bark and twigs (see Fig. 1). This chemically mediated interaction has become a model system for understanding how plant toxins affect animal physiology and behavior.

Lipophilic secondary metabolites of the plant, such as resins and essential oils, influence how much woody vegetation snowshoe hares consume in winter (Forbey et al., 2011). These fat-soluble compounds are not directly involved in plant growth but serve as protective roles. For example, resins are sticky substances that deter herbivores and protect against pathogens, while essential oils are volatile aromatic compounds that can repel herbivores or attract pollinators (see Fig. 2).

Fig. 1 Early May, a resinous twig with several leaf buds (Alaska Wildflowers, n.d.).

Fig. 2 Late April, view of birch bark with a pair of leaf buds on short twigs (Alaska Wildflowers, n.d.).

The principal component of the resin in birches is PA, a dammarane triterpene secreted by resin glands on the twigs of juvenile, resin-producing birches (e.g., Betula neoalaskana and B. pendula) (McLean et al., 2009). A triterpene is a plant chemical made of 30 carbons that often defends against herbivores or pathogens, and a dammarane triterpene is a triterpene with a specific four-ring structure (see Fig. 3). This compound acts as a chemical defense, deterring browsing by snowshoe hares (Lepus americanus) and other boreal mammals (McLean et al., 2009).

Fig. 3 Structure of papyriferic acid (McLean et al., 2009).

Snowshoe hares limit their intake of plants containing toxic secondary metabolites, like PA, to keep blood toxin levels below harmful thresholds (see Fig. 4). In experiments where only PA-rich birch twigs were available, hares ate so little that they would rapidly starve to death without access to PA-free food (Forbey et al., 2011).

Fig. 4 A snowshoe hare in Richmond changing from winter to spring coat, by Beth Comeau (Natural Resources Council of Maine, 2021).

Mode of Action of Papyriferic Acid

In animals like hares, rabbits and rats, PA uncompetitively inhibits succinate dehydrogenase (SDH), an enzyme essential for the citric acid cycle and respiratory electron transfer chain in mitochondria (Forbey et al., 2011). This inhibition disrupts cellular energy production and can harm animal health. By using malonic acid as control, a competitive inhibitor ofSDH, researchers were able to prove that papyriferic is indeed an uncompetitive inhibitor of SDH (see Fig. 5) (Forbey et al., 2011).

Fig. 5 Lineweaver–Burk plot of the effect of PA on the metabolism of succinate by ox liver mitochondria (McLean et al., 2009).

The above graph (Fig. 5) is a Lineweaver–Burk plot, used to analyze enzyme kinetics by plotting the reciprocal form of the Michaelis–Menten equation to show how PA affects SDH enzyme activity. This makes it easier to determine key enzyme parameters such as Vmax (maximum velocity) and Km (substrate affinity). As PA concentration increases, both the y-intercept (1/Vmax) and x-intercept (–1/Km) change so that the lines remain roughly parallel (McLean et al., 2009). This means both Vmax and Km decrease. This pattern indicates uncompetitive inhibition, where the inhibitor binds only to the enzyme–substrate complex, reducing both Vmax and Km by the same proportion and keeping the slope constant (McLean et al., 2009). Thus, the graph demonstrates that PA acts as an uncompetitive inhibitor of SDH

SDH is a unique enzyme of mitochondria in that it participates both in the citric acid cycle and electron transport chain (see Fig. 6). The protein is composed of four subunits (see. Fig 7). The hydrophilic head region, made up of SDHA (a flavoprotein) and SDHB (iron sulfur containing) proteins, acts as the catalytic core, while SDHC and SDHD subunits make up the membrane domain which acts as a binding site for ubiquinone (Riaz et al., 2022).

Fig. 6 Several functions of the succinate dehydrogenase in the mitochondria, involving ubiquinone (Rustin et al., 2002).

Ubiquinone, or coenzyme Q10 (CoQ10), is a fat-soluble, vitamin-like compound found in all cellular membranes. Similarly to vitamins, it acts as a coenzyme in energy production and as an antioxidant that protects cells from damage. It plays a critical role in the mitochondrial electron transport chain, facilitating efficient electron transfer, and the production of adenosine triphosphate (ATP) (Sood et al., 2024). The ubiquinone pool (UQ pool) accepts electrons from complex I (NADH dehydrogenase) and complex II (SDH) and transfers them to complex III in the electron transport chain (See Fig. 7) (Rustin et al., 2002).

Fig. 7 Strategies for targeting SDH complex (Moreno et al., 2020).

Since SDH relies on ubiquinone-binding sites to transfer electrons efficiently, interference at these sites can disrupt mitochondrial energy production. PA binds to both the proximal and distal ubiquinone-binding sites of SDH. Its carbocyclic rings form hydrogen bonds and electrostatic interactions with key residues, and its large size allows additional hydrophobic and polar contacts (Forbey et al., 2011). This binding of PA to the ubiquinone sites blocks the normal docking of ubiquinone molecules from the UQ pool, preventing the transfer of electrons from SDH to the electron transport chain. SDH has a succinate-binding catalytic site and separate ubiquinone-binding sites (Qp and Qd). Since PA binds at the ubiquinone-binding sites of SDH, it is classified as a noncompetitive inhibitor because it does not interfere with the substrate, succinate, at the catalytic site. As a result, electron flow is disrupted, leading to a reduction in ATP production. In snowshoe hares, this interference likely contributes to the antifeedant effects of PA, as the inhibition of cellular energy metabolism limits the hare’s ability to efficiently extract energy from its food. Over time, high concentrations of PA could lead to energy deficits, reduced growth, and starvation if alternative food sources are unavailable.

Geographic Mosaic of Coevolution Between Birch and Snowshoe Hare

It is worth noting that the ecological significance of PA extends beyond its molecular mechanism, as its production and efficacy vary geographically. Across the North American boreal forest, fire greatly influences snowshoe hare density (Bryant et al., 2009). Regions with frequent fires, such as Alaska, support higher hare populations because fire resets boreal forests to early successional stages. After a fire, young forests grow back with dense shrubs and saplings, especially birch, that provide abundant food and cover for hare from predators like lynx and coyotes. As a result, areas that burn more often exert strong selective pressure on juvenile-phase birches to produce higher concentrations of PA (Bryant et al., 2009). Conversely, in areas with lower fire incidence and hare density, such as New England, birch defenses are reduced. This pattern reflects a geographic mosaic of coevolution, where variation in environmental disturbance shapes the intensity of herbivory and drives corresponding variation in chemical defenses (Bryant et al., 2009). Moreover, hares from high PA regions show greater tolerance to the toxin, indicating local adaptation and illustrating the evolutionary arms race. Thus, PA, produced by juvenile birches, functions as a chemical defense against herbivory such as snowshoe hares.

Composition of Birch’s Bark and Its Hydrophobicity

Birch bark is usually taken for granted because it’s so often burned for fuel all around the world (Yadav et al., 2024). But the bark itself has remarkable chemical properties making the tree full of clever design solutions we can learn from. In this section, we will discuss the hydrophobicity of the bark. There are two main compounds responsible for the bark’s hydrophobicity: suberin and betulin. Suberin is found in specialized cells in the periderm, which is composed of the cork and the cork cambium shown below in Figure 8, and it is a biopolymer that is quite complex (Bernards, n.d.). Its chemical structure is shown in figure 9 and it is an assembly of two distinct polymeric domains, one poly(phenolic) and the other poly(aliphatic) (Bernards, n.d.). Betulin is a nonpolar molecule known as a triterpenoid that has hydrophobic properties. Suberin is the most abundant of the two, varying from 45 w/w%, or weight by weight, of the solid content in birch’s outer bark, whereas betulin is 24 w/w% (Yadav et al., 2024). Betulin’s skeletal formula is shown in figure 10 and has the following chemical formula . The numerous and bonds are what make this compound so hydrophobic. In chemistry there is a principle known as “like dissolves like”, which means that nonpolar compounds are soluble in nonpolar solvents and polar compounds are soluble in polar solvents and this ranges to an extent. This fundamental principle is what helps us explain the high hydrophobicity of birch.

Fig. 8 Anatomy of bark (Crow’s Path, 2024) (https://crowspath.org/natural-history/trees/bark/).

Fig. 9 Chemical structure of suberin (https://www.uwo.ca/biology/faculty/bernards/pdf/SuberinModel.pdf).

Fig. 10 Chemical structure of betulin (National Center for Biotechnology Information).

Betulin and Its Role in Increasing Stemflow Yields

In cloudy or foggy climates, species that have more wettable or more hydrophilic bark, benefit by soaking up and storing atmospheric water. Birch bark is hydrophobic, so it does not necessarily use this mechanism to store water and therefore resorts to other methods such as increasing stemflow. Stemflow is the portion of rainwater that runs down the trunk of a tree and gets funnelled into the soil surrounding the stem which then reaches the roots much easier. Moreover, this happens in all types of trees and not just birch (Tonello et al., 2025). In another paper by Tonello and co-authors, they compared various trees, excluding birch, and tested whether hydrophobicity of outer bark influences stemflow. They found out that trees with non-wettable bark had a stemflow yield 158% higher than wettable bark, and this indicates that these more hydrophobic species were more efficient in routing rainfall to near-stem soil, therefore framing hydrophobicity as a key factor contributing to stemflow generation (Tonello et al., 2021). Since birch is highly hydrophobic it is reasonable to assume that its bark also contributes to increasing stemflow yields. This is important for trees because it can deliver water directly to where its roots are, which increases its chance of survival in certain climates where there is not a lot of rain. This is a great design solution for birch because they thrive on cool, moist soils. They have a very shallow root system, which makes them sensitive to even short periods of drought or heating of the soil (Katovich et al., 1997). Stemflow creates a moister environment around the base of the tree by concentrating rainwater and channeling it along the shallow root system. So, this a mechanism by which birches can store water for longer periods of time while they wait for next rainfall to arrive.

Methyl Salicylate: A Protection Against Herbivores

Yellow birch, black birch, and sweet birch all contain the oil of wintergreen which is also known as birch oil. This oil is primarily composed of methyl salicylate (MeSA), which is a volatile compound, and is produced and stored in specialized glandular cells or organelles in the bark and twigs of the tree and can be released constitutively under stress (Collins, 2022; Saje Natural Wellness, 2025; Singewar et al., 2020). To get to the MeSA you would have to cut open a branch or dig through the bark for it to start coming out. Refer to fig 11 to see the chemical structure of MeSa. Interestingly, MeSA is shown to act as an inducer of plant defense against pathogens and certain piercing/sucking herbivores, particularly aphids, that are known to multiply at fast rates, colonize, and infest trees trying to get to the sap through sucking. Interestingly, MeSA is shown to reduce aphid numbers and impacts their development and fecundity (Xu et al., 2024). So, as they ingest increasingly MeSA they are more likely to experience these negative effects. MeSa can also be used for protecting maize plants against aphids as it can act as a signaling molecule when released in the air to attract the aphid’s natural predators, particularly the ladybeetle, to come eat them (Ninkovic et al., 2021; Xu et al., 2024). What this shows us is that manually adding MeSa to other plants helps them deter pests as well. In the case of yellow, black, and sweet birch, they naturally produce this chemical. When aphids try to pierce the bark or the twigs to get to the sap the methyl salicylate will get released by the cells of the tree and will negatively affect the aphids but also attract predators near by. Birch trees cannot physically fight back against the aphids trying to pierce it, so it adapted a molecule to attract nearby predators to fight for it. It follows the mechanism of “the enemy of my enemy is my friend” making for a mutualistic symbiotic relationship where the predators can benefit from getting a quick meal and the birch can benefit by getting rid of pests.

Fig. 11 Chemical structure of methyl salicylate. https://pubchem.ncbi.nlm.nih.gov/compound/Methyl-Salicylate#section=2D-Structure

Birch’s Chemical Survival Mechanisms Under Drought and Pathogen Stress

Birch has evolved a collection of chemical strategies to withstand the stresses of drought and pathogenic attacks. These natural solutions include accumulating protective metabolites, enhancing antioxidant defenses, regulating hormonal signaling for water conservation, and producing antimicrobial compounds. This section expands on these mechanisms to illustrate how chemical adjustments enable birch to survive harsh conditions.

Drought-Induced Chemical Adaptations in Birch

Drought stress triggers significant physiological and biochemical changes in birch. Among these are osmotic adjustments through solute accumulation, enhanced antioxidant enzyme activities, and hormone-mediated stomatal closure. Together, these chemical responses mitigate dehydration damage.

Osmotic Adjustment Via Compatible Solutes

Under water deficit conditions, birch trees accumulate compatible solutes (osmolytes) like proline and soluble sugars to maintain cell turgor. Experimental drought simulations have quantified substantial increases in these compounds. For instance, in European white birch (Betula pendula), PEG (polyethylene glycol)-induced drought stress caused free proline levels to rise up to about 285.9 μg/g in highly stressed seedlings with 20% PEG treatment, whereas the control group has 0% PEG concentration and is at free proline levels of about 55 μg/g (Kou et al., 2023). The proline accumulation, which was several-fold higher than unstressed controls, acts as an osmoprotectant stabilizing proteins and membranes. Even after re-watering, proline remained elevated around 263 μg/g after 20 days of recovery, indicating a lasting osmotic adjustment (Kou et al., 2023). This osmotic regulation helps birch cells retain water and cope with dehydration. In parallel, drought-exposed birch may concentrate soluble sugars like glucose and fructose as additional osmolytes. The rise in such metabolites effectively lowers the cellular osmotic potential, drawing in water to prevent excessive wilting.

Antioxidant Enzyme Defense

Drought-related oxidative stress is countered in birch by boosting antioxidant enzymes. Reactive oxygen species (ROS) accumulate during drought as water scarcity impairs normal metabolism, so birch seedlings respond by elevating enzymes that eliminate ROS, such as peroxidases, superoxide dismutase, and catalase. In one study on Betula pendula, drought stress caused a highly significant increase in these enzyme activities (p < 0.01) (Kou et al., 2023). Quantitatively, peroxidase activity in severely drought-stressed birch reached around 354.3 μmol/g fresh weight after 20 days, which was several times the activity in unstressed plants with peroxidase levels of about 42.3 μmol/g (Kou et al., 2023). Likewise, superoxide dismutase activity climbed to about 1219.89 µmol/g after 20 days of PEG-induced drought stress, and catalase peaked around 969.97 μmol/g fresh weight. These values, illustrated in Fig. 12, indicate birch’s robust antioxidant response. The elevated peroxidases, superoxide dismutase, and catalase neutralize superoxide radicals and peroxide, preventing oxidative damage to lipids and proteins. Interestingly, even at the end of the 20-day drought period with 25% PEG concentration, peroxidase and catalase levels were still rising, suggesting birch’s enzymatic defense can continuously ramp up to tolerate intensifying drought. Upon rehydration, these enzyme levels gradually declined as ROS levels normalized (Kou et al., 2023). This dynamic antioxidative adjustment highlights birch’s chemical mechanism, where it actively mitigates drought-induced oxidative stress through the action of specific enzymes.

Fig. 12 Activities of (A) peroxidase, (B) superoxide dismutase, and (C) catalase. Antioxidant enzyme activities in Betula pendula seedlings under PEG-simulated drought. Activities of ROS-eliminating enzymes increase with higher drought intensity (PEG concentration of 5-25%) and longer stress duration (10-20 days), peaking in high-stress treatments. Elevated enzyme levels indicate ramped-up ROS-scavenging capacity under drought (Kou et al., 2023).

Hormonal Signals and Stomatal Control

Plant hormones play a major role in birch drought tolerance, with abscisic acid being the primary drought signal. As soil dries, birch roots produce and release abscisic acid, which travels in the xylem to leaves and triggers stomatal closure to conserve water. In silver birch, xylem abscisic acid concentrations surge under drought. One experiment found an around 10-fold increase in abscisic acid in the xylem sap of silver birch during water limitation (Aspelmeier, 2001). This sharp rise in abscisic acid acts as an internal alarm, causing guard cells to close the stomata and reduce transpiration, thereby reducing water evaporation through the leaves. Observational data shown in Fig. 13 confirm that under severe water stress, birch leaves exhibit almost fully closed stomata (Kou et al., 2023). In a PEG-induced drought study, higher PEG concentrations led to a drastic shrinking of stomatal apertures; by 20-25% PEG treatment, most stomata were nearly closed (Kou et al., 2023). This abscisic acid-driven response cuts down water loss at the expense of photosynthesis, a trade-off that birch endures to survive drought. Abscisic acid also induces expression of stress-responsive genes and proteins that protect cells from dehydration. For example, dehydrating birch seedlings results in the accumulation of late embryogenesis abundant (LEA) proteins and dehydrins, which act as chaperones^[1] to stabilize cellular structures during water deficit. In summary, through abscisic acid signaling, birch closes its stomata and activate stress genes to minimize cell damage from drought.

Fig. 13 (CK) Control, (T1) 5% PEG, (T2) 10% PEG, (T3) 15% PEG, (T4) 20% PEG, and (T5) 25% PEG. Stomatal characteristics of birch leaves after 20 days of different PEG treatments (Kou et al., 2023).

Chemical Defense Mechanisms Against Pathogen Attacks

Birch faces biotic stress from pathogens, including fungi and bacteria. Birch has developed both constitutive and inducible chemical defenses. These include pre-formed antimicrobial compounds in bark and tissues, as well as dynamic responses such as the production of phenolic compounds, pathogenesis-related proteins, and stress enzymes when infection occurs.

Constitutive Chemical Defenses: Bark

The first line of defense is the array of antimicrobial chemicals present in birch tissues, particularly the bark. Birch bark is notably rich in pentacyclic triterpenoids like betulin (the compound that gives birch bark its white color) and betulinic acid. These compounds function as a protective shield because they have proven antifungal and antibacterial properties, helping to prevent pathogen ingress through the bark (Šiman et al., 2016). Birch outer bark can contain 20-30% betulin by dry weight, and in some species or regions, up to around 45% of the dry outer bark is betulin (Šiman et al., 2016). The high concentration of betulin in the bark is a natural chemical strategy that enables survival in pathogen-rich environments. Betulin and its derivatives disrupt fungal cell membranes and inhibit microbial growth (Fig. 14), effectively reducing rot and infection in living birch trunks (Šiman et al., 2016). In addition to betulin, birch bark contains other triterpenes like lupeol, oleanolic acid, and phenolic compounds that deter pathogens. For example, phenolic glycoside platyphylloside in Betula pendula bark can inhibit microbial enzymes (Blondeau et al., 2020). These constitutive chemicals mean that even before a pathogen attack, birch has a chemical defense at the ready. The waxy cuticle and suberin in bark further impede pathogen entry, but if pathogens do penetrate, they encounter these toxic chemicals that can slow or stop infections in the wood and phloem.

Fig. 14 Visual representation of the ways betulinic acid can disrupt microbial growth (Hamion et al., 2024).

Induced Chemical Responses to Infection

When birch detects a pathogen attack, it intensifies its chemical defenses by producing additional metabolites and proteins. When silver birch is infected with the pathogenic fungus Phytophthora plurivora, birch responds by upregulating heat shock proteins (HSPs) and altering secondary metabolite production. Specifically, the expression of Hsp90, a molecular chaperone, is spiked seven times higher than baseline in infected birch trees that are under 60% defoliation stress (Berezovska et al., 2021). This dramatic increase (measured via gene expression levels) indicates activation of the tree’s immune-related proteins; Hsp90 and related Hsp83 likely help refold proteins and stabilize signaling molecules during pathogen attack, thereby enhancing resistance (Berezovska et al., 2021). The HSPs are part of the plant’s systemic acquired resistance, ensuring cells can cope with the stress of infection. Beyond protein-based defenses, birch also produces certain chemical compounds in response to pathogenic attack.

When birch faces both pathogen infection (Phytophthora plurivora) and heavy defoliation (a combined stress scenario), it produces the highest flavonoid content in its tissues, while moderate stress had somewhat lower levels (Berezovska et al., 2021). Flavonoids, such as catechin, luteolin derivatives, and quercetin-like compounds, were detected only in the heavily stressed samples, suggesting that birch synthesizes or accumulates these phenolic compounds in response to the pathogen. Flavonoids are known as antimicrobials and antioxidants; in birch, they likely help limit fungal growth at infection sites and mitigate oxidative bursts caused by pathogen invasion. The stressed birch (60% defoliation) showed a 2.5-fold increase in sterols and fatty acids (Fig. 15) in its tissues compared to control trees (uninfected and non-defoliated) (Berezovska et al., 2021). This increase in membrane lipids and sterols could strengthen cell barriers and enhance the production of signal molecules during an attack. Additionally, the combination of defoliation stress with Phytophthora infection led to a marked rise in triterpenes (the same class as betulin) within the birch shoots (Berezovska et al., 2021). Some specific triterpenoids (dammarane-type and ursane-type acids) were found at their highest concentrations under pathogen pressure, hinting that birch actively increases these defensive chemicals to inhibit the pathogen’s spread (Berezovska et al., 2021). The accumulation of these compounds is a form of chemical immune response, where many triterpenes and phenolics can directly deter pathogens or bolster the structural integrity of cell walls, for example, by facilitating lignin synthesis, which physically blocks pathogen advancement.

Fig. 15 Comparing the levels of sterol and fatty acid between the control (first column from the left) and the 60% defoliation (third column from the left) (Adapted from Berezovska et al., 2021).

Moreover, birch trees under pathogen attack often activate pathogenesis-related (PR) proteins such as chitinases and glucanases that break down fungal cell walls. These proteins are part of the induced chemical defense by signaling pathways involving salicylic acid which is a hormone that typically rises during biotic stress. Therefore, through both increased secondary metabolites and protein defenses, birch chemically defends itself against pathogens.

Synergy and Trade-offs

Birch’s response to combined stresses like drought and pathogens can be complex. Moderate stress can sometimes prime birch defenses. For example, a mild defoliation stress actually counteracted some root damage from Phytophthora plurivora, resulting in birch trees with healthier roots than those affected by the pathogen alone (Berezovska et al., 2021). This suggests a cross-talk where drought-related signals might pre-activate certain defense pathways related to pathogen defenses (a phenomenon known as stress priming). On the other hand, severe abiotic stress might exhaust the tree and reduce its capacity to fight pathogens. Birch tries to balance its chemical responses. In the study by Berezovska et al. (2021), (Berezovska et al., 2021) birch under the dual extreme of heavy defoliation and pathogen attack had high defense metabolite production (flavonoids, triterpenes), yet the total damage was still harmful. The finding underlines that birch’s chemical defenses are adaptive, where the tree will boost different compounds depending on the nature and intensity of stress. For instance, an insect or fungal attack on birch leaves is known to induce the production of more tannins and phenolic glycosides, which make the foliage unpalatable or inhibit the growth of pathogen enzymes (Blondeau et al., 2020). If the pathogen is vascular, like Phytophthora in roots, birch invests in root-zone defenses and general stress proteins (ex. HSPs), which help stabilize damaged cells and slow the pathogen’s spread through root tissues. By protecting the roots, which serve as the tree’s primary water and nutrient highway, these chemical defenses help maintain survival even while infection is ongoing (Berezovska et al., 2021).

Birch Resistance to Herbivore Pressure by Secondary Metabolites

Birches, like nearly all plants, are autotrophs. They create their own glucose from photosynthesis, a process primarily occurring in the leaves of the tree. Photosynthetic tissue is one of the tree’s most important structures, but also its most vulnerable. Leaves are soft and rich in vital nutrients, a primary food source for forest-dwelling herbivores and insects. The damage that can be done to leaves by insects is pictured on Fig. 16. As primary sites of photosynthesis, leaves are immense potential sources of energy and require significant energy investment from the tree to grow. While supporting the diversity of herbivores in forest ecosystems is also important for the birch tree’s survival, the birch must also manage its energy efficiency and protect itself from overgrazing.

Fig. 16 Birch leaves being damaged by insect feeding (Townsend, 2020).

Trees are sessile organisms and therefore lack the ability to escape predation or the harm of pests (Xie et al., 2025). Instead, in encountering these natural pressures, plants evolved inducible and constitutive defenses. Constitutive defense mechanisms refer to permanent physical characteristics of plants that deter pests and grazers, such as thorns, hairs, bark, waxy cuticles, and spines (pictured in Fig. 17), or perpetually maintained chemical characteristics such as toxins, tannins, and irritants like capsaicin. Alternatively, trees also utilize inducible defenses that are activated in response to stimuli, such as grazing or insect herbivory (War et al., 2012). This includes proteins and secondary metabolites— organic chemical compounds that form a reactionary defense (Xie et al., 2025).

Fig. 17 Trichome thorns, an example of physical constitutive defense in roses (Courtesy of Awkward Botany, 2022). Retrieved from https://awkwardbotany.com/tag/trichomes/.

The Jasmonic Acid Pathway

There are several reasons why inducible chemical defenses that utilize secondary metabolites are favorable as opposed to constitutive chemical defenses. Maintaining the growth and diversity of the local ecosystem is beneficial for birch trees, as herbivores are useful pollinators and increase forest productivity (Siemens et al., 2010). For this reason, trees evolved defense systems that are more tolerant and optimal for resource allocation, rather than absolutely inhibiting all herbivore activity. It is a waste of energy to constantly maintain constitutive defense compounds in every part of the tree, whereas inducible and localized responses to harm are far more energy efficient (Siemens et al., 2010).

A vital and prominent secondary metabolite for defense against insect pests is jasmonic acid (structure pictured in Fig. 18) and its associated compounds in birch (Xie et al., 2025). These precursors and derivatives are referred to as jasmonates (Ruan et al., 2019). Like many organic compounds, jasmonates are multipurpose, used for signalling responses to wounding, microbial pathogens, herbivorous insects, and low temperature stress (Ruan et al., 2019).

Fig. 18. Chemical structure of jasmonic acid and its functions. In birch trees, it is primarily a signaling molecule (Courtesy of TriumphIAS, 2020). Retrieved from https://triumphias.com/blog/jasmonic-acid/.

Jasmonic acid is not stored or constantly produced, but rather completely synthesized from scratch in response to stimuli. When tree cells are damaged by the mechanical stress of insect chewing or certain microbial and fungal pathogens, a polypeptide precursor called prosystemin is hydrolyzed into systemin, a much smaller signalling molecule that binds to hormone receptors that kickstart the jasmonic acid pathway (Ruan et al., 2019). Jasmonic acids are derived from lipids and can be synthesized using the fatty acids of damaged cell membranes. This localized, induced process conserves energy by only activating the pathway, when necessary, in the affected region. Additionally, the birch tree can further regulate and adjust tolerance to herbivore grazing. By increasing the concentration of the hormonal precursor prosystemin in active young leaves and decreasing concentration in older leaves, the tree can preferentially resist damage to more important photosynthetic tissue (Xie et al., 2025).

Jasmonic acid is synthesized and released through the extracellular matrix and spreads throughout the tree. Jasmonic acid hormone signals the production of defensive secondary metabolites, such as phenols and terpenoids (War et al., 2012). Plant phenols are a widespread and common class of large polymers, found in birch trees and specified to defend against insects. For example, lignin, a phenolic compound found in cellulose physically bolsters the structure of the cell. It increases leaf toughness to reduce feeding by insects and herbivores and physically blocks the entry of microbial or fungal pathogens into birch leaf cells (War et al., 2012). Tannins are a key group of phenolic compounds in birch that bind to digestive enzymes in insects. Tannins are bitter tasting and have low palatability, inhibiting the digestive ability of feeders, making birch leaves less palatable (War et al., 2012). On a more extreme scale of defense, birch trees also produce terpenoids and other toxic, insecticidal, antifungal, and antimicrobial compounds in response to the release of jasmonic acid. These toxic, volatile organic compounds can even act as pheromones to attract the predators of feeding insects (War et al., 2012).

Jasmonic acid is a remarkable chemical, a vital part of an ingenious natural defense mechanism in birch trees. Beyond signaling the release of defense metabolites, it even regulates growth and regeneration of damaged structures (War et al., 2012). This exploration into the tree’s natural biochemical defense systems illustrates how the tree masterfully manages resources, reusing destroyed cell structures to synthesize response hormones and fine-tuning these pathways to optimize the tree’s energy efficiency and grazing tolerance. Nature is an expert and diversifies the use of single components to amazing lengths—this single compound and the metabolites synthesized on its signal have a plethora of effects that extend beyond the scope of this section. Regardless, these examples illustrate how in every one of nature’s design problems and solutions, organisms additionally balance the overarching forces of survival, reproduction, growth, and resource management. The birch tree has evolved spectacularly adaptable chemical pathways that stand up to the complex interplay of ever-changing internal and external conditions that influence the survival of the tree.

Conclusion

In conclusion, birch trees exhibit remarkable biochemical complexity which contributes to their relentless survivability. Birch trees face intense herbivory pressure from snowshoe hares, which feed on their twigs, especially in young post-fire forests where hares are abundant. To defend themselves, juvenile birches produce PA. At the biochemical level, PA functions as a noncompetitive inhibitor of SDH, binding to the ubiquinone sites, and disrupting mitochondrial electron transport and ATP production. This interference reduces the energetic reward hares gain from feeding birch twigs, making the plants less desirable as food. Birch trees thrive in cool environments and moist soil. Due to its bark’s increased hydrophobicity, which is due to the betulin embedded in it, birch can increase stemflow yields and keep the soil moister for longer. Birch trees and trees in general are affected by aphid infestations from time to time. Methyl Salicylate protects birch from aphid infestations by affecting their fecundity and development and serves as a signaling molecule to attract the natural predators of aphids to come eat them, effectively reducing their population. When facing drought and pathogen attacks, birch counters them through chemical defenses. They conserve water with osmoprotectants and abscisic acid signaling, while compounds like betulin and flavonoids protect against microbes and strengthen cell structures. Finally, when faced with extensive herbivory, the birch tree can produce innumerable protective organic chemicals to protect vital photosynthetic tissue. These pathways are only activated in response to localized stimuli rather than constantly maintained, conserving energy by only activating these defenses when needed. These compounds are masterfully applied to extensive and distinct utilizations in the tree to support its survival, a testament to the tree’s adaptability. The birch tree is a physical marvel, special in its ability to withstand and occupy the ruthless subarctic niche.